Having a jammed horn that won't turn off is a sticky situation at best. You don't want to get on the road and cause a total nuisance and attract unnecessary attention to yourself, and you also don't want the horn to kill your battery. Should you ever find yourself in a situation where your car horn is jammed, there are several remedies you can use until you get the problem repaired.
When a horn gets stuck, something in the steering column is often jammed. Before you try anything else, push on the horn with a lot of force and twist the steering wheel back and forth several times. This may release whatever is sticking, and shut off the jammed horn. If this doesn't work you will have to try something else.
The next thing you should do is locate your owner's manual and determine whether your horn has its own fuse. Many car models have a fuse specifically for the horn; the fuse diagrams in your owner's manual will show you where the fuse is. If your horn does have its own fuse, then take a pair of needle nose pliers and remove that fuse. Be careful, though, some car horns share a fuse with something else that will also lose power if you disconnect the horn.
If your horn is indeed sharing a fuse with something else, then your next option is to physically unhook the power from the horn. Car horns typically look like a donut or a loud speaker, though they do vary by model. Locate the car horn, disconnect the battery, and then disconnect the power from the car horn. Then wrap the red wire lead from the horn with electrical tape. This will prevent an accidental short circuit. Reconnect the battery and the jammed horn will be off.
If you cannot locate the horn, then your last option is to just disconnect the battery. Open the hood and disconnect the red lead from the battery. This will turn the horn off, but your car will not operate. This is less than ideal, but at least the horn will be off until you can identify the problem.